A major problem encountered in the treatment of automotive exhaust gas treatment concerns the so-called “cold start” period of the treatment process, when the exhaust gas as well as the exhaust gas treatment system have low temperatures. At these temperatures, catalytic exhaust gas treatment systems do not display sufficient activity for effectively treating hydrocarbon, NOx and/or CO emissions. As a result, considerable efforts have been made to alleviate this problem, in particular by developing trapping systems which store emissions at low temperatures and subsequently release them at higher temperatures, at which catalytic components present in the system have attained a sufficient activity for treating them.
Thus, trapping materials have been developed for retaining specific emissions during the cold start period of automotive combustion, wherein hydrocarbons and NOx have received the most attention in view of environmental concerns. For ease of implementation, multi-component exhaust gas treatment articles have been developed which aim at combining the various catalytic and trapping activities into as few elements as possible. As a result of this, a large number of products incorporate both catalytic and trapping activities, for example by adopting a multilayer structure wherein the different functions are located in the different layers.
Regarding NOx-trapping components, for example, there is a tendency in their design to combine them with a hydrocarbon trap capable of catalyzing selective catalytic reduction. Thus, JP 11226415 A and JP 11300211 A respectively disclose a NOx storage catalyst comprising a first layer on a substrate which contains a nitrogen oxide storage material, and a second layer provided on the first layer which contains a hydrocarbon trapping material supporting a selective catalytic reduction catalyst. EP 935055 A, on the other hand, additionally teaches the introduction of an intermediate layer to these layers, wherein said layer essentially consists of alumina and/or silica and is free of noble metal, for improving the thermal stability of the active components of the first and third layers. However, the combining of different functions in such multi-component systems often leads to unwanted interactions between the individual functionalities. In particular, it has been found that combining hydrocarbon trapping and selective catalytic reduction functionalities into the same component of a multilayer system affords poor results with respect to the conversion of NOx.
On the other hand, multi-component systems exist which incorporate hydrocarbon and NOx trapping without including elements or compounds capable of catalyzing selective catalytic reduction in the hydrocarbon trapping material. Thus JP 2005169203 A discloses a multilayer NOx trap containing a hydrocarbon trap layer on a substrate and upper layers disposed on the hydrocarbon trap layer which contain a nitrogen oxide storage material and a selective reduction catalyst. Said NOx traps, however, display a decreased conversion rate with respect to hydrocarbons, NOx and CO during the cold start period of exhaust gas treatment compared to NOx traps devoid of the hydrocarbon trapping functionality.
It would be desirable to provide an improved NOx storage catalyst, as well as an improved method for the treatment of automobile exhaust gas and an improved treatment system for an automobile exhaust gas stream. In particular, it would be desirable to provide an NOx storage catalyst with improved hydrocarbon conversion activity which does not impair the catalyst's activity to convert CO and NOx.